For many years, the depth effect of stereoscopic images has exerted great fascination on viewers. This effect is obtained by the left eye seeing an image that is horizontally offset from the image seen by the right eye. When looking directly at three-dimensional objects, holograms, or 3D images provided with lens arrays, the offset between the images seen in each eye is due to the physical offset between the optical axes of the eyes. Unfortunately, those techniques cannot be implemented for looking at printed images suitable for being widely distributed. Other stereoscopic systems use a first image for the left eye which is made invisible to the right eye by first masking means, and a second image for the right eye which is made invisible to the left eye by second masking means.
In stereoscopic slide viewers, which are typically fitted with two identical converging lenses forming two eyepieces, an opaque partition lying in the sagittal plane between the eyes and the slides acts as masking means by preventing the left eye from seeing the image for the right eye, and vice versa.
The physical offset of the two images of the slide requires a viewer that is relatively complex and expensive in order to look at the stereoscopic image, which viewer must also be adapted to the format of the slides to be looked at. Since there is no single image, there is no simple way of looking at a two-dimensional image (without depth).
In another type of known device, masking means are provided by spectacles comprising a first polarizing filter having a first polarization placed in front of the left eye and a second polarizing filter having a second polarization that is crossed relative to the first placed in front of the right eye, thereby enabling the two eyes to see different images either simultaneously or alternately.
Polarizing filters can be replaced by shutters, e.g. liquid crystal shutters, placed in front of the eyes, with a first shutter being transparent when the second is opaque and vice versa, with the shutter cycles lasting, substantially one-thirtieth of a second, for example. To enable stereoscopic images to be seen, a computer system monitor displays images for the left eye interlaced with images for the right eye. Equipment that is complex and expensive is needed to see such stereoscopic images.
In the above cases, from the moment the pictures are taken, it is necessary to acquire two images: respectively for the left eye and for the right eye. Making a stereoscopic image from a plane image, e.g. a conventional photograph, is impossible, or at any rate extremely complex.
Finally, in known manner, the masking means may comprise a red filter placed in front of the left eye and a blue filter placed in front of the right eye. The image to be looked at comprises either a blue image for the right eye superposed on a red image for the left eye, thereby generating a good depth effect, or else a color image in which the foreground subject has red and blue fringes of considerable width on its left and right sides, thereby generating a moderate depth effect. The quality of such images is insufficient to enable the plane images (i.e. without any depth effect) to be acceptable when looked at without using colored spectacles.
U.S. Pat. No. 4,558,359 describes a method of transforming monochrome images into stereoscopic color images by coloring the image and by adding colored elements which are offset relative to the original image.
The Applicants have discovered that stereoscopic images are looked at mainly without using spectacles, particularly when they are being looked at for selection purposes. With the exception of three-dimensional images covered in lens arrays, which technique and high price have not enabled widespread distribution, present techniques do not make it easy to look at images without using additional equipment, in particular when selecting images to be looked at and/or purchased.